Flotation is a process commonly used for the separation of dispersed particulate matter from slurries or suspensions and also for the separation of oily substances from emulsions, typically water based. The flotation process relies on particle collection by attachment to air bubbles deliberately dispersed in the suspension. Collection results from particle surfaces being either naturally hydrophobic or rendered selectively hydrophobic by conditioning with appropriate reagents. Bubbles with attached hydrophobic particles subsequently rise under their natural buoyancy to form a surface froth layer which is removed from the residual suspension. Typically, after a certain residence time, almost all hydrophobic particles are removed. Surfactants are usually added in the case of mineral slurries to facilitate formation of a stable froth on the slurry surface.
An important objective of any flotation apparatus is to disperse the introduced gas as finely as possible to maximise the bubble population and to create an environment in which there is a high probability of successful particle/bubble collision. The apparatus must also have a quiescent zone which allows bubbles to separate from the gasified pulp and coalesce to form a froth on the pulp surface, for subsequent removal. The various forms of known apparatus can be divided into three categories with respect to the mechanism of particle/bubble collision. In the first category, particle/bubble contacting is conducted at gravitational acceleration by cruising bubble collision. This type of contacting is known to lose its efficiency as particles get smaller. The rate of particle/bubble collision in these processes is relatively low so a long residence time is required to yield satisfactory recovery. Typical apparatus in this category includes all purely pneumatic cells and conventional flotation columns.
In the second category, particle/bubble contacting is conducted by precipitation of gas on the hydrophobic surface. Typical apparatus in this group includes dissolved air flotation systems.
In the third group of apparatus, particle/bubble collision occurs mostly at accelerations much higher than gravitational, leading to better collision efficiency due to the effect of inertial impaction. Inertial impaction at high acceleration levels is an important mechanism for enhancing particle/bubble collision, particularly for very fine particles which are known to have very low collision rates at gravitational acceleration levels due to their tendency to follow flowlines around the bubble. Higher acceleration levels increase the inertia of the fine particles so they depart, from the liquid flowline and the probability of their collision with bubbles substantially increases. The generation of shear rates required for effective gas dispersion in flotation devices usually results in high accelerative fields, but such fields are seldom created by deliberate design, as they are usually a by-product of a gas dispersing or solids suspension technique. This applies in the case of gas dispersion by intensive impeller agitation, widely used in commercial types of mechanical flotation cells, dispersion by crossflow gas injection as in the Bahr cell [1], dispersion in swirling nozzles as in the Davcra cell [2], and dispersion by a plunging jet into a column of slurry as in the Jameson cell [3]. Only the gas sparged hydrocyclone [4] conducts flotation in a deliberately high centrifugal accelerative field with gas crossflow. In this device gas is injected through a porous cylindrical wall of the hydrocyclone into rapidly moving slurry having a spiral motion. The resulting high liquid shear rate generates small bubbles which move quickly through the slurry due to high accelerative forces and collide with hydrophobic particles in the process. Unfortunately this device is rather energy demanding and its performance is hindered by blockages of the porous septum. The gas sparged hydrocyclone is further disadvantaged in the case of minerals beneficiation by its limited ability to produce high grade concentrates. It produces finely structured froth which is rapidly removed from the device, offering little opportunity for froth drainage normally required for grade improvement, nor is froth washing physically possible in the device to improve concentrate grade.
All of the above known devices have a disadvantage in that the acceleration intensities that can be achieved are limited, either because the device was not designed with this parameter in mind, or because maintaining high acceleration intensities would lead to excessive energy consumption or wear problems of moving or stationary parts exposed to high velocity-abrasive slurry flows. For example, in impeller flotation machines the gas cannot be dispersed effectively unless the impeller speed is high, and this leads to high power input and substantial wear problems. Therefore, to enhance the separation of fine particles in particular, flotation technology would clearly benefit from an apparatus which is designed to generate very high shear rates and accelerative fields without the associated wear problems and excessive energy consumption as encountered in current known technology.
It is the purpose of the present invention to provide a simple, efficient and economic means to improve the flotation process by creating in a flotation apparatus, a mixing zone with both the very high shear rate necessary for fine gas dispersion and accelerative and decelerative fields which improve the probability of particle bubble contacting, but without any moving or complex stationary parts being exposed-to high velocity flows of abrasive slurry. It is a further purpose of the invention to be able to introduce conditioning chemicals if required, via a liquid jet, as well as to utilize a novel froth crowding technique to obtain higher concentrate grades without the necessity of froth washing.